In the production of bleached kraft pulp there have been increasing regulatory and market demands to reduce or eliminate the presence of chlorinated organic compounds in pulp products and bleaching effluents. In order to properly react to such demands, it is necessary to eliminate the use of chlorine gas or any other chlorine containing compound. However the removal of the chlorine based bleaching agents makes it extremely difficult--in fact impossible under present technology--to achieve desired brightness levels, especially if the pulp produced is to have acceptable strength properties. For example oxygen delignification must be utilized, and multiple stage oxygen delignification--especially with chelating treatment to control deleterious metal ions--has been shown to offer advantages in delignification and selectivity, especially when there is between stage washing (see U.S. Pat. No. 4,946,556). However there are practical limits governing both the extent of delignification and the quality of pulp obtainable using oxygen delignification alone.
Of other available bleaching agents, perhaps the most promising is ozone. It has been demonstrated that ozone is a highly effective delignifying agent, however ozone has seen only marginal commercial acceptance to date in the pulp industry. The main obstacles to commercial utilization of ozone have been the chemical cost of ozone when large doses are needed, and the degradation of pulp strength that typically occurs relative to conventionally bleached pulps.
Thus, there has long been a need for bleached kraft pulp with commercially acceptable strength properties without the use of chlorine based bleaching agents. According to the present invention, it is possible to finally achieve that goal. This is accomplished according to the present invention by utilization of ozone in order to take advantage of its powerful delignifying effect, while limiting the amount of ozone applied to the pulp to less than 1%, and while holding pulp degradation to a minimum.
According to the present invention, the basic approach that is taken is to have a pulp with minimum Kappa Number and maximum strength before it is subjected to ozone bleaching sequence. This is accomplished according to the present invention by utilizing kraft pulp produced from extended delignification processes. Continuous digesters sold by Kamyr, Inc. of Glens Falls, N.Y. under the trademark "MCC" have, since 1988, been utilized to produce under a process known as modified continuous cooking a strong softwood kraft pulp with a Kappa Number of about 23 to 25 (this compares with the Kappa Number of about 30-32 achieved in conventional kraft ("CK") pulping of softwood). A variation of the modified continuous cooking process, practiced in digesters sold by Kamyr, Inc. of Glens Falls under the trademark "EMCC", practice a process known as extended modified continuous cooking. Such a process can produce softwood pulps having an even lower Kappa Number, typically 18-20, while maintaining a high pulp viscosity--comparable to that for a CK pulp at a Kappa Number of 30.
Another procedure that may be utilized to produce extended delignification pulp--allowing the production of high brightness, high strength bleached pulp without chlorine based bleaching agents--is to subject CK pulp to multiple stage oxygen delignification, with between stage washing, such as described in the above-mentioned U.S. Pat. No. 4,946,556. While the pulp produced according to this aspect of the invention has lower brightness and strength properties than pulp produced as set forth above, they are still within the commercially acceptable range.
The extended delignification pulp is, according to the invention, subjected to ozone bleaching, with an ozone dosage of less than 1.0% on pulp by weight (and preferably less than about 0.75% and most desirably less than about 0.5%), to produce a chlorine free bleached pulp with brightness of about 90 CPPA or greater. While a number of ozone bleaching sequences may be acceptable, one that is particularly advantageous is that described in co-pending application Ser. No. 07/721,780, filed Jun. 28, 1991. The bleaching sequence shown therein is (ZE)P(ZE)P, the (ZE) stages being ozone followed by extraction without washing between them. There is washing between the (ZE) and P stages. This bleaching sequence--especially if preceded by a pretreatment stage to remove metal ions, and two oxygen stages with between stage washing--produces excellent brightness (well over 90 CPPA) pulp, with good strength properties. Even though the pulp so produced has viscosity lower than CK pulp, it has been found that the lower viscosity does not mean less strength, and the T+2B value for pulp according to the invention is comparable to chlorine compound bleached VK pulp.
According to one aspect to the present invention, a method of continuously kraft pulping and then bleaching comminuted cellulosic fibrous material utilizing an upright digester having top, bottom, and central portions is provided. The method comprises the steps of: (a) Passing comminuted cellulosic fibrous material entrained in kraft white liquor into the top of the digester. (b) Extracting black liquor from at least one screen between the top and bottom of the digester. (c) At a first portion of the digester withdrawing and recirculating liquid in a first recirculation loop. (d) Adding kraft white liquor to the first recirculation loop. (e) Adjacent the bottom of the digester withdrawing and recirculating liquid in a second, wash, recirculation loop. (f) Withdrawing kraft pulp from the bottom of the digester, steps (a)-(e) being practiced to produce pulp having a Kappa Number comparable to about 25 or below for softwood. And, (g) ozone bleaching the kraft pulp with an ozone dosage of less than 0.1% on pulp by weight, to produce a chlorine-free bleached pulp with brightness of about 90 CPPA or greater. The procedure just described is Kamyr, Inc. 's modified continuous cooking process (hereafter "M"), typically practiced in an MCC.TM. digester. The method can include the further step of adding kraft white liquor to the second recirculation loop, in which case Kamyr, Inc. 's extended modified continuous cooking (hereafter "E") process is practiced, typically in a Kamyr EMCC.TM. digester.
According to another aspect of the present invention, a method of producing bleached kraft pulp is practiced comprising the following steps: (a) Subjecting kraft pulp to two stage oxygen delignification with between stage washing to produce pulp having a Kappa Number comparable to about 12 or less for softwood. And, (b) ozone bleaching the oxygen delignified kraft pulp with an ozone dosage of less than 1.0% on pulp by weight, to produce a chlorine-free bleached pulp with brightness of about 90 CPPA or greater.
It is the primary object of the present invention to provide for the production of kraft pulp having a brightness of greater than 90 CPPA and commercially acceptable strength, without utilizing chlorine containing bleaching compounds, and with minimal use of ozone. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.